Meeting nutritional needs in critically ill patients is a primary concern in their management and recovery. Nitrogen balance for these patients by adequate protein intake through enteral/parenteral nutrition is a key aim. How much Nitrogen intake and the Amino Acid (AA) composition fo this protein in patients with acute renal failure (ARF) is not clearly established. Adjustments in the protein intake of the patient for the form of RRT (renal replacement therapy) is often necessary. An increasingly common form of RRT in ARF is Continuous Venovenous Hemofiltration (CVVH). Two forms are commonly used: CVVH with FRF (filtered renal failure) and CVVHD (continuous venovenous diafiltration). This project's objective will be to evaluate pediatric patients on CRRT for Amino Acid Clearance by both modalities (CCVH + FRF vs. CVVHD) to determine its effect on nitrogen balance.